This invention relates in general to fiber optics, and in particular, to an optical mode conditioner for efficiently conditioning a single mode optical signal propagating in a single mode optical fiber or launched from a single mode laser transmitter for propagation within a multimode optical fiber.
Fiber optic data links for many computing platforms have traditionally been one of two varieties, i.e., multimode fiber or single mode fiber. With a single mode fiber, light is launched down the axis of the fiber, and with a multimode fiber light propagates down the axis and also bounces back and forth at many different angles within the fiber. Multimode fiber has a larger core size, for example, 50 or 62.5 xcexcm in diameter, compared with single mode fiber, for example, 9-10 xcexcm in diameter. Further, multimode fibers are typically used for shorter distances, up to 3 km, while single mode fiber enables transmission of signals across longer distances, for example, up to 20 km or more. Because of the different core sizes, there are conventionally two different types of fiber optic transceivers, one for each cable type.
Recently, however, there have been a number of applications which require a single mode transceiver/card to operate over a multimode fiber. For example, in a Parallel Sysplex for an IBM S/390 system, the channel coupling links were initially offered in both multimode and single mode versions, however the multimode adapters were subsequently withdrawn. Therefore, to facilitate customers who had multimode adapters installed and wish to migrate to single mode, it is desirable to be able to reuse the installed multimode fiber with a single mode adapter card. As another example, Gigabit Ethernet is a new IEEE industry standard (no. 802.3Z) which specifically provides for operation of single mode transceivers over multimode fiber optic under certain conditions. As a further example, FICON is an IBM implementation of a fiber channel intended as replacement for ESCON, now an industry standard. Since the vast majority of ESCON links installed today use multimode fiber, it is desirable to provide a way to reuse the fiber with single mode-only FICON adapter cards, similar to the Gigabit Ethernet standard.
To address these needs, it is necessary to have a means of operating a single mode laser card over multimode fiber. A single mode laser signal cannot be directly launched into multimode fiber since the optical power will not be distributed evenly among all the modes and, a type of noise develops known as differential mode delay (DMD), which prevents the link from operating at any distance more than a few meters. In order to achieve useful distances, the signal from the single mode card must be adapted to the multimode fiber. This adaption is conventionally done using an optical mode conditioning patch cable (MCP).
The present invention is directed to providing an improved optical mode conditioning technique for implementation within an MCP for conditioning a single mode waveform for propagation within a multimode fiber.
Briefly described, the invention comprises in one aspect an optical mode conditioner which includes a single mode fiber optic section having a diameter x and a flared end with a diameter y, wherein y greater than x. The flared end is configured so that in operation a single mode signal propagating within the single mode fiber optic section is expanded within the flared end to an equilibrium mode distribution for propagation within a multimode fiber optic section to be coupled to the flared end. In an enhanced embodiment, the conditioner further includes a multimode fiber optic section having an end aligned to the flared end of the single mode fiber optic section. In one embodiment, the flared end of the single mode fiber optic section and the aligned end of the multimode fiber optic section are fusion spliced together. Further, the single mode fiber optic section may comprise any polymer material suitable for use as a fiber optic, i.e., a material that is transparent at an operational wavelength of the fiber optic channel.
In another aspect, a mode conditioning patch is provided which includes a single mode optical fiber section and a multimode optical fiber section. The single mode optical fiber section has a diameter x and a flared end, with the flared end having a diameter y, wherein y greater than x. The multimode optical fiber section has an end disposed in opposing relation to the flared end of the single mode optical fiber section. The flared end of the single mode optical fiber section is fabricated to expand a single mode signal propagating within the single mode optical fiber section to an equilibrium mode distribution for propagation within the multimode optical fiber section.
In a further aspect, the invention comprises a method for conditioning a single mode optical signal for propagation in a multimode optical fiber. The method includes: passing the single mode optical signal from a single mode optical fiber section, through a transition region optic and into a multimode optical fiber section, wherein the single mode optical fiber section has a diameter x and the multimode optical fiber section has a diameter Y, with y greater than x, and the single mode optical fiber section, the transition region and the multimode optical fiber section have an aligned axis; and wherein the transition region optic expands the single mode optical fiber signal to an equilibrium mode distribution for propagation within the multimode optical fiber section.
To restate, provided herein is an optical mode conditioning technique and mode conditioning patch which allows significantly smaller mode conditioners to be fabricated as compared with conventional MCPs. These significantly smaller mode conditioners do not measurably change an existing optical link length, and thereby maintain a constant delay through a fiber channel irrespective of whether a single mode or multimode channel is in use. Further, provided herein is an MCP adapter which is significantly less expensive to fabricate compared with a conventional MCP cable employing ceramic offset ferrules. Also, since there are no ceramic offset ferrules in the technique presented, no active alignment is needed.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered part of the claimed invention.